As part of its new data plan offerings for smartphone users, AT&T is at long last letting iPhone users in on the action with a tethering option.

Here’s how it works.

iPhone tethering will only be available for users who sign up for AT&T’s DataPro plan. Under the Data Pro plan, users get 2GB of data for $25/month. If these users want to get their tethering on, they’ll have to shell out an additional $20/month. All told, that comes out to $45/month for tethering+data. Keep in mind, that this price is strictly for data, and doesn’t take into account fees for voice. Lastly, we should point out that users on AT&T’s DataPro plan (whether they use tethering or not) will be charged an additional $10 for each 1GB of data they use over the 2GB threshold.

Tethering for the iPhone will go live when Apple releases iPhone OS 4 later this Summer.